XHR requests from web-based ACME clients provide the User-Agent
of the browser that initiated the request, but the hostname of the site
that originated the request is sent in the Origin header. This will let
us better analyze web-based ACME traffic.
Fixes#4370
In the VA, we were rendering a Duration to JSON, which gave an integer
number of nanoseconds rather than a float64 of seconds. Also, in both VA
and WFE we were rendering way more precision than we needed. Millisecond
precision is enough, and since we log latency for every WFE response,
the extra bytes are worth saving.
Roland Bracewell Shoemaker
Jacob Hoffman-Andrews